Blue Rubicon will deliver a 12-month creative brand-building campaign to engage both new and existing users in a bid to increase market share for the service, which launched in print in the 1990s and moved operations online in 2004.
The agency will report to Jason Leonard, head of Soulmates, and was appointed following a pitch.
Leonard said it was the first time an agency had been called in for profile building.
"2014 will be an important and busy year for Guardian Soulmates," he added, "and we wanted to appoint an agency that would deliver a creative campaign that captures the public’s imagination."
Online dating has been a growth market in recent years, with reports pointing to nine million Britons using dating sites and research firm Mintel estimating the market will be worth £150m by next year.